The present invention relates generally to ion implantation, and more particularly to the use of polished graphite as Einzel lens electrodes for reducing electrostatic discharge of high voltage during an ion implantation process in semiconductor manufacturing.
Ion implantation uses charged particles (ions) to penetrate beneath a material""s surface, which gives the material unique electronic, mechanical, or chemical properties. Ion implantation is deemed as a key technique in the semiconductor industry. It is also used in other manufacturing sectors for its demonstrated potential for hardening of surfaces and for enhancing the corrosion properties of metals.
Within the microelectronics industry , ion implantation techniques are used to introduce impurity atoms into semiconductors to alter the conductivity of the semiconductors in a controlled fashion. In ion implantation, electrically charged ions are accelerated under the action of an electric field and implanted into a solid target, i.e., a semiconductor wafer. Along with ions of the desired species, implanters sometimes inadvertently deposit contaminants onto wafer surfaces. These contaminants may be in the form of particles or ions and molecules of another species. The contaminants may be produced by the ion source and transported through the beamline, or generated by sputtering caused by energetic ions impinging on surfaces in the beamline, or caused by electrostatic discharges (arcing) within other components of the ion implantation system. Various implantation systems with capabilities for high current, high energy and low-energy ion implantation are available commercially, such as those manufactured by Axcelis Technologies, Inc., Applied Materials, Inc., and Varian Semiconductor Equipment Associates, to name a few.
One of the possible components of the optics system of an ion implantation system is an electrostatic lens, commonly called an Einzel lens, which aids in focusing the ion beam, and, in some cases, can be used for deceleration optics. Einzel lenses have electrodes which may be made of metal, metallic compounds, or high-purity graphite. Metal electrodes for Einzel lenses are not used for semiconductor devices (wafers) because undesirable metal contamination generated from the electrodes could result.
Generally, graphite Einzel electrodes are employed on ion implanters in instances when low energy (typically 10 keV or below) beams are extracted from the ion source, and where low contamination content is a requirement, as in the semiconductor manufacturing industry. The use of graphite electrodes eliminates the problem of metal contamination, however, standard high-purity graphite electrodes have surface irregularities such as peaks or apexes which exist on the surface of the graphite. Since tens of kilovolts may be applied to the electrodes during an implant operation, these surface irregularities often lead to electrostatic discharges (arcing) from the electrodes, which results in high particulate contamination of the implanted wafers. In some cases, wafers damaged by an electrostatic discharge cannot be repaired due to the physical nature and extent of the damage. In addition, moisture may be trapped within the surface irregularities present on the electrode, which creates an additional impurity problem.
These problems are generally addressed by conditioning the electrodes (under vacuum) through techniques such as photoresist outgassing, or the use of an argon beam through the lenses to remove moisture and other impurities which may be present on the surface of the graphite. However, this conditioning process typically takes more than one week, during which time the implantation system is unavailable for low energy production work.
Given the aforementioned limitations of conventional Einzel lenses used in ion implantation systems, it is apparent that an Einzel lens which overcomes the particulate contamination problems without rendering the implantation device unavailable for extended periods of time is needed.